On 12/30/25 15:06, Gregory Price wrote:
On Tue, Dec 30, 2025 at 10:55:02AM +0800, Huang, FangSheng (Jerry) wrote:
Hi Gregory,
Sorry i've missed prior versions, is numa the right place to put this,
considering that the node is not necessarily 100% SPM on a real system?
The decision to add `spm=` to NUMA rather than the memory backend was based
on
earlier feedback from David during our initial RFC discussions.
David raised a concern that if we put the spm flag on the memory backend, a
user
could accidentally pass such a memory backend to DIMM/virtio-mem/boot
memory,
which would have very undesired side effects.
This makes sense, and in fact I almost wonder if we should actually
encode a warning in linux in general if a signal NUMA node contains
both normal and SPM. That would help drive consistency between QEMU/KVM
and real platforms from the direction of linux.
Yeah, in theory we would have a "memory device" for all boot memory
(boot DIMM, not sure ...) and that one would actually be marked as "spm".
It's not really a thing of a memory backend after all, it's only how
that memory is exposed to the VM.
And given we don't have a boot memory device, the idea was to set it for
the Node, where it means "all boot memory is SPM". And we only allow one
type of boot memory (one memory backend) per node in QEMU.
The tricky question is what happens with memory hotplug (DIMMs etc) on
such a node. I'd argue that it's simply not SPM.
(in practice it should be, but not technically required to be)
You're right that on a real system, a NUMA node is not technically required
to
be 100% SPM. However, in AMD's use case, the entire NUMA node memory (backed
by
memdev) is intended to be SPM, and this approach provides a cleaner and
safer
configuration interface.
I figured this was the case, and honestly this just provides more
evidence that any given NUMA node probably should only have 1 "type" of
memory (or otherwise stated: uniform access within a node, non-uniform
across nodes).
That makes sense.
---
bit of an aside - but at LPC we also talked about SPM NUMA nodes:
https://lore.kernel.org/linux-mm/[email protected]/
Would be cool to be able to detect this in the drivers and have hotplug
automatically mark a node SPM unless a driver overrides it.
(MHP flag? Sorry David :P)
:)
If it's a per-node thing, MHP flags feel a bit like "too late". It
should be configured earlier for the node somehow.
~Gregory
Please let me know if you have further concerns or suggestions.
I'll look at the patch details a bit more, but generally I like the
direction - with an obvious note that I have a biased given the above.
Thanks for taking a look!
--
Cheers
David
